1. Field of the Invention
The present invention relates to a rotary actuator having a rotor which is rotatable relative to a stator.
2. Discussion of Prior Art
A rotary actuator has been utilized in association with a diverter gate in the sorting of mail or other items traveling by a conveyor. The rotary actuator is effective to rotate the diverter gate from one position to another position within a matter of a few milliseconds, typically within about 0.020 seconds, so as to permit a rapid sorting process. The angle of rotation of the diverter gate is typically about 15° to 20° to move the item of mail from one conveyor path to another conveyor path.
The angle of rotation through which the diverter gate is moved is limited by rubber stop bumpers. The rubber stop bumpers are mounted external to the rotary actuator so as to allow precise adjustment and to minimize impact noise by the diverter gate. At the end of its operating stroke, the diverter gate may tend to rebound as it impacts against one of the rubber bumpers.
If the diverter gate can rebound back into the previous conveyor flow path, a missortment or jam may occur. To prevent a missortment or jam from occurring, the flow rate of mail or other items must be decreased to give time for the diverter gate to return to its fully actuated position. Alternatively, the rate of operation of the rotary actuator must be decreased to reduce the kinetic energy of the rotary actuator and diverter gate at an end of stroke position. Of course, both of these solutions to the problem of diverter gate rebound are counter to rapid sorting requirements.
The rotary actuator for the diverter gate must provide for both rapid movement of the diverter gate from an unactuated position to an actuated position and holding of the diverter gate at its actuated position upon impact of the diverter gate against a rubber bumper. In order to provide both functions adequately, the starting torque of the rotary actuator must be high to provide a high diverter gate acceleration rate. The ending torque of the rotary actuator must be high to counteract the rebound energy imparted by the rubber bumper to the diverter gate.
Known rotary actuators have previously utilized either one of two basic design approaches. The first basic design approach utilizes a pole configuration termed as “constant air gap” for the rotor and stator pole pieces. The second basic design approach utilizes a pole configuration termed as “diminishing air gap” for the rotor and stator pole pieces. The air gaps are the working air gaps across which magnetic flux is conducted between the rotor and stator pole pieces.
The “constant air gap” rotary actuator design is characterized by a high starting torque that decreases to a lower torque as the rotary actuator operates through its operating stroke (it being assumed that a constant current is applied to the coil of the rotary actuator). The high starting torque occurs when lobes of the rotor are only partially overlapping, or aligned with, corresponding stator lobes. Typically, there is a 3° overlap of the rotor lobes and stator lobes at the initial starting position of the rotor.
The maximum torque for the “constant air gap” rotary actuator design occurs between the initial position and an overlap position of about 10°. The torque then steadily drops off for the remainder of the stroke. For a rapid response, a high starting torque is essential to overcome inertia of components of the rotary actuator and diverter gate. However, a rotary actuator of the “constant air gap” design has a relatively low torque at the end of its operating stroke. This relatively low torque is insufficient to prevent rebound of a diverter gate upon impacting of the diverter gate against a rubber bumper.
The “diminishing air gap” rotary actuator design is characterized by a relatively low starting torque due to large initial air gaps between the rotor and stator pole pieces at the beginning of the operating stroke of the rotary actuator. As the rotor rotates, the air gaps decrease and the torque steadily rises toward a high ending torque. Therefore, for a given power level and loading conditions, the rotary actuators having a “constant air gap” design will produce a higher starting torque than the rotary actuators having a “diminishing air gap” design. However, the “diminishing air gap” rotary actuator design will have a higher end of stroke torque. Although the “diminishing air gap” rotary actuator design has potential to have a relatively high ending torque, small variations in the final position of the diminishing air gaps, being in a series magnetic circuit arrangement, can result in a large variation in the end of stroke torque of the “diminishing air gap” rotary actuator design.
In the foregoing discussion of the background of the present invention, the rotary actuators have been considered in association with a diverter for mail or other items that are traveling along a conveyor. It should be understood that rotary actuators have and, in all probability, will be used in many different environments. For example, rotary actuators have previously been utilized to actuate valves which control fluid flow.